1. Field of the Invention
The present invention relates to a method and apparatus for transmitting an uplink scheduling request in a mobile communication system.
2. Description of the Related Art
A multiplexing scheme employed in wireless communication may be divided into a time division multiplexing scheme, a code division multiplexing scheme, an orthogonal frequency division multiplexing scheme and so forth. A multiplexing scheme in most general use today is the code division multiplexing scheme, which is in turn divided into a synchronous scheme and an asynchronous scheme. However, the code division multiplexing scheme suffers from lack of resources because it basically uses codes and thus orthogonal codes are insufficient. Thereupon, the Orthogonal Frequency Division Multiplexing (hereinafter OFDM) scheme is now in the spotlight.
The OFDM scheme, one of data transmission schemes using a multi-carrier, is a type of Multi-Carrier Modulation (hereinafter MCM) scheme in which a serial input symbol stream is converted into parallel symbol sub-streams, the converted symbol sub-streams are modulated with multiple sub-carriers (i.e., multiple sub-carrier channels) orthogonal to each other, and then the modulated symbol sub-streams are transmitted. The OFDM scheme is similar to a Frequency Division Multiplexing (hereinafter FDM) scheme, but is different from the FDM scheme in that orthogonality between multiple sub-carriers is maintained during transmission, and frequency spectrums are overlappingly used. Thus, the OFDM scheme is efficient in the use of frequencies, is robust to frequency selective fading and multi-path fading, and can reduce the effect of inter-symbol interference (hereinafter referred to as “ISI”) by using a guard interval. Further, the OFDM scheme can provide optimal transmission efficient in high-speed data transmission because it makes it possible to simply design the structure of an equalizer in hardware and has an advantage of high resistance to impulse noise.
The 3GPP (3rd Generation Partnership Project) is currently discussing a next generation mobile communication system as a substitute for the Universal Mobile Telecommunication Service (hereinafter UMTS) system that is the 3rd generation mobile communication standard. Such a next generation mobile communication system is called a Long Term Evolution (hereinafter LTE) system.
FIGS. 1A and 1B illustrate an example of a UMTS-based wireless mobile communication system, that is, a 3GPP LTE system, to which reference is made in the present invention.
Referring to FIG. 1A, a user equipment (hereinafter referred to as “UE”) 11 is a terminal for the 3GPP LTE system, and an Evolved Radio Access Network (hereinafter E-RAN) 14 is a radio base station equipment directly involved in communication with a terminal in an existing 3GPP system. The E-RAN 14 serves not only as a node B for managing cells, but also as a Radio Network Controller (hereinafter RNC) for controlling a plurality of node Bs and radio resources. With regard to this, the E-RAN 14 may include an Evolved Node B (hereinafter E-NB) 12 and an Evolved RNC (hereinafter E-RNC) 13 that are physically separated into different nodes, as in an existing 3GPP system, or integrated into one node. By way of example, it is assumed herein that the E-NB 12 and the E-RNC 13 are physically integrated into one node. However, it is obvious that the present invention can be applied in the same manner even if the E-RNC 13 is physically separated from the E-NB 12.
An Evolved Core Network (hereinafter E-CN) 15 is a node into which functions of a Serving GPRS Support Node (hereinafter SGSN) and a Gateway GPRS Support Node (hereinafter GGSN) in an existing 3GPP system are combined. The E-CN 15 is located between a Packet Data Network (hereinafter PDN) and the E-RAN 14, and serves as a gateway for allocating an Internet Protocol (hereinafter IP) address to the UE 11 and connecting the UE 11 with the PDN 16. Since definitions and functions of the SGSN and the GGSN follow the standards specified in 3GPP, a detailed description thereof will be omitted herein.
Referring to FIG. 1B, an Evolved UMTS Radio Access Network (hereinafter E-RAN) 110 has a simplified two node structure of an Evolved Node B (hereinafter E-NB) 120, 122, 124, 126, 128 and an anchor node 130, 132. A User Equipment (hereinafter UE or terminal) 101 is connected with an IP network via the E-RAN 110. Each E-NB 120 to 128 corresponds to an existing Node B of the UMTS system, and is connected with the UE 101 over a radio channel. Dissimilar to the existing Node B, the E-NB 120 to 128 performs more complex functions. In the LTE system, since all user traffics including a real-time service through an IP, such as a Voice over IP (hereinafter VoIP) service, are serviced via a shared channel, an apparatus for collecting and scheduling situation information of UEs is needed. The E-NB 120 to 128 performs the function of such an apparatus.
In general, one E-NB controls a plurality of cells, and Adaptive Modulation & Coding (hereinafter AMC) for determining a modulation scheme and a channel coding rate in conformity with the channel state of a UE is performed in an E-NB. Further, similar to High Speed Downlink Packet Access (hereinafter HSDPA) or High Speed Uplink Packet Access or HSUPA; also called Enhanced-uplink Dedicated Channel or E-DCH of the UMTS system, the LTE system uses Hybrid Automatic Retransmission Request (hereinafter HARQ) between the E-NB 120 to 128 and the UE 101. However, since various Quality of Service (hereinafter QoS) requirements cannot be satisfied by the HARQ alone, an upper layer may perform a separate ARQ (hereinafter outer-ARQ), which also takes place between the UE 101 and the E-NB 120 to 128. The HARQ refers to a technique for increasing a reception success rate by soft-combining retransmitted data with previously received data without discarding the previously received data, and is used for improving transmission efficiency in high-speed packet communication, such as the HSDPA. In order to enable a transmission speed of maximum 100 Mbps, the LTE system is expected to employ the OFDM scheme as radio access technology with a bandwidth of 20 MHz.
FIG. 2 illustrates a procedure in which a UE transmits a scheduling request or buffer status report to an E-NB, based on 3GPP HSUPA technology.
If data or control signals to be transmitted by a UE is generated when no resource is allocated to the UE, the UE transmits a scheduling request or buffer status report to an E-NB in order to request the E-NB to allocate radio resources for transmitting the data or control signals.
The scheduling request and buffer status report are different only in name, and are substantially the same. That is, they correspond to a procedure for informing the E-NB of information on the priorities of data or control signals to be transmitted, the amounts of the data or control signals according to priorities, filled in a buffer, etc. in order to request the E-NB to allocate radio resources for transmitting the data or control signals in uplink. Upon receiving the scheduling request or buffer status report from the UE, the E-NB allocates radio resources to the UE. In the present invention, the scheduling request is abbreviated as “SR”, and the buffer status report is abbreviated as “BSR”. The SR or BSR may be transmitted by PHY (PHYsical) signaling or MAC (Medium Access Control) signaling.
Referring to FIG. 2, if data or control signals to be transmitted are generated in each of UE#1, UE#2, . . . , and UE#N 201, 202, 203 by an upper layer, each of the UE#1, the UE#2, . . . , and the UE#N 201, 202, 203 transmits an SR/BSR 231, 232, 233 to an E-NB 211. Upon receiving the SR/BSRs 231, 232, 233 from the UEs, the E-NB 211 generally allocates radio resources to each of the UE#1, the UE#2, . . . , and the UE#N 201, 202, 203 when a cell is not loaded with radio resources. The radio resources allocated to each of the UE#1, the UE#2, . . . , and the UE#N 201, 202, 203 are determined by information included in the SR/BSR 231, 232, 233 transmitted from each of the UEs, that is, the corresponding priorities of the data or control signals to be transmitted and the amounts of the data or control signals according to priorities, filled in a buffer.
In FIG. 2, requested radio resources cannot be allocated to all of the UEs because a cell is loaded with radio resources, as designated by reference numeral 221. If the E-NB 211 receives the SR/BSRs 231, 232, 233, but cannot allocate radio 25 resources to all of the UE#1, the UE#2, . . . , and the UE#N 201, 202, 203, IDs of the UEs and radio resource allocation information are not included in scheduling information transmitted in the downlink. If the UEs 201, 202, 203 cannot be allocated with radio resources for uplink transmission from the scheduling information transmitted in the downlink after transmitting the SR/BSRs 231, 232, 233, they retransmit SR/BSRs 241, 242, 243 to the E-NB 211. This is performed on the assumption that the E-NB 211 fails to receive the SR/BSRs 231, 232, 233 transmitted by the UEs. However, at a point of time when the UEs retransmit the SR/BSRs 241, 242, 243 after transmitting the SR/BSRs 231, 232, 233, information on the priorities of the data or control signals to be transmitted by the UEs and the amounts of the data or control signals according to priorities, filled in a buffer, may vary, and thus the SR/BSRs 241, 242, 243 may include different values from those in the SR/BSRs 231, 232, 233.
Although the E-NB 211 receives the SR/BSRs 241, 242, 243, it may not allocate radio resources for uplink transmission, requested by the UE#1, the UE#2, . . . , and the UE#N 201, 202, 203, because a cell is loaded with radio resources, as the time when the E-NB 211 receives the SR/BSRs 231, 232, 233. If IDs of the UEs and radio resource allocation information are not included in scheduling information transmitted in downlink after the SR/BSRs 241, 242, 243 are transmitted, the UEs can know that radio resources for uplink transmission are not allocated.
If the UE#1, the UE#2, . . . , and the UE#N 201, 202, 203 cannot be allocated with requested radio resources for uplink transmission after transmitting the SR/BSRs 241, 242, 243, they retransmit SR/BSRs 251, 252, 253 to the E-NB 211. If the E-NB 211 does not allocate radio resources to the UEs even after receiving the SR/BSRs 251, 252, 253, the UEs 201, 202, 203 retransmit SR/BSRs 261, 262, 263 to the E-NB 211. That is, if the UEs cannot be allocated with radio resources for uplink transmission after transmitting SR/BSRs, they repeatedly retransmit the SR/BSRs. However, when the E-NB 211 does not allocate radio resources for uplink transmission to the UEs not because it fails to receive the SR/BSRs, but because a cell is loaded with radio resources irrespective of successful reception of the SR/BSRs, not only the repetitive SR/BSRs 241, 242, 243, 251, 252, 253, 261, 262, 263 are of no use, but also many problems, including inefficient consumption of radio resources and power consumption of UEs, and unnecessary interference may be caused.